Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are better than traditionally used silicon, germanium, or gallium arsenide. Diamond provides a higher energy band gap, a higher breakdown voltage and a greater saturation velocity than these traditional semiconductor materials. These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using conventional semiconductor materials. For example, silicon is typically not used at temperatures higher than about 200.degree. C. and gallium arsenide is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for silicon (1.12 eV at ambient temperature) and gallium arsenide (1.42 eV at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 eV at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments. In other words, diamond is also a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond for high temperature and radiation-hardened electronic devices. Key to many of such devices, such as diodes and field effect transistors (FET's), is a rectifying contact having good rectifying characteristics even at relatively high temperatures. Consequently, the fabrication of rectifying contacts on diamond will play an important role in the development of future diamond-based semiconductor devices.
Rectifying contacts have been successfully obtained on single crystal semiconducting diamond. For example, U.S. Pat. No. 4,982,243 to Nakahata et al. discloses a rectifying contact on a single crystal diamond layer that may be formed by the chemical vapor deposition of refractory metals, such as tungsten, molybdenum, niobium, tantalum, as well as other materials such as aluminum, polycrystalline silicon, nickel, gold, platinum, tungsten carbide, molybdenum carbide, tantalum carbide, niobium carbide, tungsten silicide or molybdenum silicide. Unfortunately, to produce a rectifying contact, a single crystal diamond substrate must be used and a single crystal diamond layer must be homoepitaxially deposited on the substrate. Moreover, the single crystal diamond substrate must have a polished surface which inclines at an angle of not larger than 10.degree. to a (100) plane.
Similarly, European patent application No. 417,645 A1 to Shiomi, entitled MES Field Effect Transistor Formed in a Diamond Layer, discloses a rectifying gate electrode requiring single crystal diamond. The rectifying gate electrode may be gold, platinum, copper, molybdenum, tungsten, aluminum, nickel cobalt, manganese, or carbides thereof. However, a single crystal diamond substrate is relatively expensive and large substrate sizes are not readily available as desirable for many semiconductor applications.
Geis, in an article entitled High-Temperature Point-Contact Transistors and Schottky Diodes Formed on Synthetic Boron-Doped Diamond, IEEE Electron Device Letters, Vol. EDL.-8, No. 8, pp. 341-343, August 1987, discloses a point contact rectifying contact for a transistor on single crystal diamond. Similarly, Shiomi et al. in Characterization of Boron-Doped Diamond Epitaxial Films and Applications for High-Voltage Schottky Diodes and MESFET's, New Diamond Science and Technology, 1991 MRS Int. Conf. Proc., pp. 975-980, discloses a titanium rectifying contact formed on a homoepitaxial diamond film deposited on a (100) surface of a synthesized single crystal type Ib diamond substrate.
U.S. Pat. No. 5,155,559 to Humphries et al. entitled High Temperature Refractory Silicide Rectifying Contact and Method for Making Same, discloses a rectifying contact on either single crystal or polycrystalline diamond. The contact includes a layer of refractory metal silicide. Moreover, a non-abrupt interface region is formed between the metal silicide and diamond by annealing the as-deposited metal silicide. The non-abrupt interface region may thus include silicon carbide, the carbide of the refractory metal, and mixtures thereof.
Surface roughness and grain boundaries provide reverse current leakage paths and degrade contact performance for polycrystalline diamond. Metals deposited onto polycrystalline diamond in an attempt to form a rectifying contact may diffuse into the diamond film through grain boundaries during subsequent high temperature processing or during operation, thereby degrading performance.
While polycrystalline diamond is a preferred material for many semiconductor devices, its surface roughness and grain boundaries have impeded the development of rectifying contacts on polycrystalline diamond. These disadvantages are further highlighted at higher temperatures where diffusion is more likely to occur. Accordingly, the advantages using polycrystalline diamond, such as lower cost and high operating temperatures, have not been realized.
Despite the lesser expense and relative ease of fabricating polycrystalline diamond films as compared to single crystal diamond, attempts to obtain good rectifying characteristics on polycrystalline diamond have had little success. For example, Gildenblatt et al. in an article entitled High Temperature Schottky Diodes with Boron-Doped Homoepitaxial Diamond Base, Mat. Res. Bul., Vol. 25, pp. 129-134 (1990), report attempts to fabricate Schottky contacts using gold and aluminum on polycrystalline diamond, but indicate that such contacts have shown unacceptably high leakage current above 250.degree.-300.degree. C. Accordingly, Gildenblatt et al. teach a homoepitaxial diamond film formed on a single crystal diamond substrate to provide a rectifying contact with a gold electrode layer.
An article by Miyata et al. entitled Metal-Insulator-Semiconductor Diodes Using Polycrystalline Diamond Thin Films, presented at the ECS Meeting in Washington, D.C., in 1991, discloses that improved rectification of a diamond rectifying contact was obtained by providing an undoped diamond layer between a semiconducting diamond layer and an aluminum layer.